Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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Dual stack optical data storage medium and use of such medium
The invention relates to a dual-stack optical data storage medium for at
least read out using a focused radiation beam with a wavelength ? between
400 nm and an Numerical Aperture (NA) between 0.84 and 0.86, entering through
an
entrance face of the medium during read out, comprising:
- a substrate with present on a side thereof:
- a first stack of layers named L0, comprising a first information layer,
- a second stack of layers named L1, comprising a second information
layer, L1 being present at a position closest to the entrance face and LO more
remote
from the entrance face than L1,
- a radiation beam transparent spacer layer between LO and L1,
- a radiation beam transparent cover layer between the entrance face
and L1
- a transmission stack named TSO with a thickness dTSO and an effective
refractive index nTSO containing all layers between LO and the entrance face,
- a transmission stack named TS1 with a thickness dTSl and an effective
refractive index nTS1 containing all layers between L1 and the entrance face.
The invention also relates to the use of such medium.
In one aspect of the present invention, there is provided a dual-stack
optical data storage medium for at least read out using a focused radiation
beam with
a wavelength a, between 400 nm and 410 nm and an Numerical Aperture (NA)
between 0.84 and 0.86, entering through an entrance face of the medium during
read
out, comprising: a substrate with present on a side thereof: a first stack of
layers
named LO comprising a first information layer, a second stack of layers named
L1,
comprising a second information layer, L1 being present at a position closest
to the
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entrance face and LO more remote from the entrance face than L1, a radiation
beam
transparent spacer layer between LO and L1, a radiation beam transparent cover
layer between the entrance face and L1, a transmission stack named TSO with a
thickness dTSO and an effective refractive index nTSO containing all layers
between LO
and the entrance face, a transmission stack named TS1 with a thickness dTSl
and an
effective refractive index nTS1 containing all layers between L1 and the
entrance face,
wherein the spacer layer has a thickness selected from the range 20-30 pm, the
thickness dTSO in dependence on the refractive index nTSO is within the upper
shaded
area in Fig. 1 and the thickness dTs1 in dependence on the refractive index
nTS1 is
within the lower shaded area in Fig. 1.
An embodiment of such an optical recording medium is known from a
paper "New Replication Process Using Function-assigned Resins for Dual-layered
Disc with 0.1 mm thick Cover layer", by K. Hayashi, K. Hisada and E. Ohno,
Technical Digest ISOM 2001, Taipei, Taiwan. A minimum spacer layer thickness
of
30 pm was disclosed.
There is a constant drive for obtaining optical storage media suitable for
recording and reproducing, which have a storage capacity of 8 Gigabyte (GB) or
larger. This requirement is met by some Digital Video Disk or sometimes also
Digital
Versatile Disk formats (DVD). DVD formats can be divided into DVD-ROM that is
exclusively for reproduction, DVD-RAM, DVD-RW and DVD+RW, which are also
usable for rewritable data storage, and DVD-R, which is recordable once.
Presently
the DVD formats comprise disks with capacities of 4.7 GB, 8.5 GB, 9.4 GB and
17 GB.
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The 8.5 GB and, in particular, the 9.4 GB (DVD-9) and 17 GB (DVD-18)
formats exhibit more complicated constructions and usually comprise multiple
information
storage layers. The 4.7 GB single layer re-writable DVD format is easy to
handle
comparable, for example, to a conventional compact disk (CD) but offers an
insufficient
storage capacity for video recording purposes.
A high storage capacity format that recently has been suggested is Digital
Video Recording (DVR). Two formats are currently being developed: DVR-red and
DVR-
blue, the latter also called Blu-ray Disc (BD), where red and blue refer to
the used radiation
beam wavelength for recording and reading. This disk overcomes the capacity
problem and,
in its simplest form, has a single storage layer format which is suitable for
high density
digital video recording and storage having a capacity above 22 GB in the DVR-
blue format.
The DVR disk generally comprises a disk-shaped substrate exhibiting on one
or both surfaces an information storage layer. The DVR disk further comprises
one or more
radiation beam transmissive layers. These layers are transmissive to the
radiation beam that is
used to read from or write into the disk. For example a transmissive cover
layer, which is
applied on the information storage layer. Generally, for high-density disks,
lenses with high
numerical aperture (NA), e.g. higher than 0.60, are used for focusing such a
radiation beam
with a relatively low wavelength. For systems with NA's above 0.60 it becomes
increasingly
difficult to apply substrate incident recording with substrate thicknesses in
the 0.6-1.2 mm
range due to decreasing tolerances on e.g. thickness variations and disk tilt.
For this reason,
when using disks that are recorded and read out with a high NA, focusing onto
a recording
layer of a first recording stack, is performed from the side opposite from the
substrate.
Because the first recording layer has to be protected from the environment at
least one
relatively thin radiation beam transmissive cover layer, e.g. thinner than 0.5
mm, is used
through which the radiation beam is focused. Clearly the need for the
substrate to be radiation
beam transmissive no longer exists and other substrate materials, e.g. metals
or alloys
thereof, may be used.
A dual-stack optical storage medium has two reflective information layers,
that
are read-out from the same side of the medium. In this dual stack medium case,
where a
second recording stack is present, a radiation beam transmissive spacer layer
is required
between the recording stacks. The first recording stack must be at least
partially transparent
to the radiation beam wavelength in order to make reading from the recording
layer of the
second recording stack possible. The thickness of such spacer layers typically
is thicker than
30 m. The radiation beam transmissive layer or layers which are present
between the
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radiation beam source and the recording stack that is most remote from the
substrate are
normally called cover layers. When prefabricated sheets are used as
transmissive layers extra
transmissive adhesive layers are required in order to bond cover layers to
each other.
In the DVR disk the variation or unevenness of the thickness of the radiation
beam transmissive layers over the radial extension of the disk has to be
controlled very
carefully in order to minimize the variation in the optical path length for
the impinging
radiation. Especially the optical quality of the radiation beam at the focal
point in the BD or
DVR-blue version, which uses a radiation beam with a wavelength substantially
equal to 405
urn and an NA substantially equal to 0.85, is relatively sensitive to
variations in the thickness
of the transmissive layers. The total layer thickness has an optimal value in
order to obtain
minimum optical spherical aberration of the focused radiation beam on, e.g.,
the first
information recording layer. A deviation, e.g. +/- 5 m, from this optimal
thickness already
introduces a considerable amount of this kind of aberration. Because of this
small range it is
important that the average thickness of the transmissive layers is equal to or
close to its
optimal thickness in order to make optimal use of the tolerances of the system
and to have a
high yield in manufacturing the medium. Assuming that a thickness error is
Gaussian
distributed around the nominal setting of the thickness, it is clear that the
number of
manufactured disks which do not comply with the above specification is minimal
when the
target setting of the nominal thickness during manufacture is substantially
equal to the
optimal thickness of the cover layer as in the specification of the DVR disk.
Two studies of the spacer-layer thickness were published recently for DVD dual-
layer discs.
A numerical aperture of 0.6, readout through the substrate of 0.58 mm and
light of 405 Mn
wavelength were used. An optimum spacer layer thickness of 30 um has been
found by Lee
et al. Jpn. J. Appl. Phys. Vol 40 (2001) pp 1643-1644 and 40 pm was found by
Higuchi and
Koyanagi, Jpn. J. Appl. Phys. Vol. 39 (2000) 933 [4].
For a system with 0.1 mm thin cover layer and a high NA of 0.85 and a
wavelength of 405
urn additional correction of the spherical aberration (proportional to 2JNA)
is required. To
neglect the interference from the neighboring layer a spacer layer of
minimally 30 m has
been considered necessary. This has the disadvantage that the drive design for
reading out
such a medium in such case has to be rather complicated in order to cover the
necessary
range for spherical aberration correction. Further the cover layer of such
medium may
become relatively thin and the underlying layers are more susceptible to
damage.
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It is an object of some embodiments of the invention to provide a
medium of the kind as described in the opening paragraph with a reliable read
out of data from the first information layer and form the second information
layer.
This object is achieved in accordance with the invention by an optical data
storage medium which is characterized in that the spacer layer has a thickness
selected from
the range 20 - 30 m, the thickness drsO in dependence on the refractive index
nTSO is within
the upper shaded area in Fig.1 and the thickness dTsI in dependence on the
refractive index
nTSI is within the lower shaded area in Fig. 1. The specifications of the
Transmission Stacks
(TS) include all possible layers on top of the concerning recording stack,
such as e.g. gluing
layers in case of foils, the spacer layer and the semi-transparent recording
stack of Li in case
of TSO, the Cover Layer and possibly a Protective coating). From EP-A-1047055
it is known
to use a polymer layer such as, for example, a polycarbonate (PC) sheet as
light-transmissive
cover or spacer layer and adhere such layer to the information storage layer
by means of a
thin, spin-coated layer of a UV curable liquid resin or a pressure sensitive
adhesive (PSA).
To find the minimal spacer-layer thickness for the blue system with high NA
the dependence of the data quality was studied when read out from the medium
as function of
the spacer-layer thickness. It was found that in general, the spacer layer
thickness or the
amount of separation of the first information and the second information layer
depends on the
size of the photo-detector in the optical pick-up unit (OPU) of the optical
medium drive, the
magnification from the photo-detector to the medium, the reflectivity ratio of
the first and
second information layers and the distance between the two layers, i.e. the
thickness of the
spacer layer. A stable OPU design restricts the size of the photo detector and
the
magnification of objective lens and collimator lens. Tolerance for aging and
alignment errors
require a minimum detector size of 100 m and a magnification of about 10.
First the
influence of the stray-light on the recording performance has been modeled.
The main
influence comes from reduction of the signal modulation resulting in a
decrease
of the signal to noise ratio. In a second step, the amount of stray-light as
function
of the spacer-layer thickness is simulated using ray tracing.
In an embodiment the maximum deviations of dTso and dTs I from respectively
the average values of d1so and dTSI between a radius of 23 mm and 24 mm of the
medium do
not exceed 2 m measured over the whole area of the medium. This has the
advantage that
no substantial correction for spherical aberration is required when the first
information layer
of the medium or the second information layer of the medium is scanned by the
optical
medium drive. During scanning the OPU will move radially inward our radially
outward
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while the medium rotates. When the thickness variations of TSO and TSl are
within said
limits also the spherical aberration stays within acceptable limits over the
whole area of the
medium. The only instance when correction is required is when the OPU switches
from
focusing onto the first information layer to focusing onto the second
information layer or vice
5 versa.
In another embodiment nTSO and nTsI both have a value of 1.6 and the
following conditions are fullfilled: 95 m <d.TSO <_105 j m and 70 m SdTSI
<_80 m. Most
plastic materials used as transparent layers have a refractive index of 1.6 or
substantially
close hereto. In this case reliable read out is possible when the thicknesses
fall within the
mentioned ranges.
In a further embodiment the spacer layer thickness is 25 m or substantially
close to 25 m and the cover layer thickness is 75 m or substantially close
to 75 m. It is
advantageous from a viewpoint of manufacture to use a substantial fixed value
of the spacer
and cover layer thickness. For instance, one method of manufacture comprises
the application
of a pressure sensitive adhesive (PSA) with a predetermined thickness which is
UV cured
after being brought in contact with other layers of the medium. This material
is usually
supplied as a sheet of foil with the PSA on one or sides and those sheets are
made with a
predetermined thickness.
The invention will be elucidated in greater detail with reference to the
accompanying drawings in which
Fig. 1 shows the allowable area of thickness of the transmission stacks TSO
and TS 1 as a function of the refractive index.
Fig. 2 schematically shows the layout of a dual-stack recording medium
according to the invention.
Fig. 3 shows a simulation of data-to-clock jitter when read out as function of
the stray-light from the adjacent, out-of-focus, information layer.
Fig. 4 shows a ray-tracing simulation of the light reflected onto the photo-
detector as function of the spacer-layer thickness.
In Fig.1 the allowed thickness ranges of TSO and TS1 are indicated. The
thickness dTsO in dependence on the refractive index nTsO is within the upper
shaded area 1
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and the thickness dTSI in dependence on the refractive index nTSI is within
the lower shaded
area 2. The spacer layer 24 (Fig. 2) has a thickness selected from the range
20 - 30 m.
In Fig.2 an embodiment of the dual-stack optical data storage medium 20
according to the invention is shown. A focused laser beam 29 with a wavelength
X of 405 nm
and an Numerical Aperture (NA) of 0.85 enters through entrance face 26 of the
medium 20
during read out. A substrate 21 made of polycarbonate has present on a side
thereof-
a first stack of layers 22 named LO comprising a first information layer, a
second stack of
layers 23 named L1, comprising a second information layer. L1 is present at a
position
closest to the entrance face 26 and LO is present more remote from the
entrance face 26 than
Ll. A transparent spacer layer 24 made of a UV cured resin, e.g. SD 694 made
by DIC, is
present between LO and Ll. A transparent cover layer 25 is present between the
entrance face
26 and Ll and may be made of the same material or a sheet of PC or PMMA with a
pressure
sensitive adhesive (PSA). The spacer layer may also be a sheet combined with
PSA. The
transmission stack named TSO has a thickness dTSO of 100 m and an effective
refractive
index nTso =1.6 and contains all layers between LO and the entrance face 26.
The Ll stack 23
has a relatively low thickness of a maimally a few hundred nm the influence of
which may be
neglected. Naturally L1 does affect the optical transmission but this aspect
is not dealt with
here. The transmission stack named TS i has a thickness dTSI of 75 pm and an
effective
refractive index nTSI of 1.6 and contains all layers between L1 and the
entrance face (26). The
spacer layer (24) has a thickness of 25 m. The thickness dTso =100 m at a
refractive index
nTso = 1.6 falls within the upper shaded area in Fig.l and the thickness dTsI
= 75 m at a
refractive index nTSO 1.6 falls within the lower shaded area in Fig. 1.
In Fig. 3 the modeled data-to-clock jitter in %, when reading the first
information layer of LO, as function of the stray-light from the out of focus
layer, e.g. the
second information layer of Ll, is represented by graph 30. The jitter without
stray-light was
chosen to be 5.8%. At a stray-light level of 15% the jitter has increased from
5.8% to 6.5%
which is tolerable.
In Fig. 4 the ray-tracing simulation of the light reflected onto the photo-
detector as function of the spacer-layer thickness is represented by graph 40.
A 15% upper
limit on the stray-light is represented by dotted line 41. The stray-light as
function of the
spacer- layer thickness was calculated for a OPU detector size of 100 m and a
magnification
factor of 10. The minimum spacer layer 24 (Fig. 2) thickness to guarantee less
than 15%
stray-light is 20 m.
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According to the invention a dual-stack optical data storage medium is
described for read out using a focused radiation beam with a wavelength of 400
- 410 nm and
a Numerical Aperture (NA) of 0.84 - 0.86. The medium has a substrate and a
first stack of
layers named LO comprising a first information layer and a second stack of
layers named Ll,
comprising a second information layer. A radiation beam transparent spacer
layer is present
between LO and L1. A transmission stack named TSO with a thickness dTso and an
effective
refractive index nTSO contains all layers between LO and an entrance face of
the medium. A
transmission stack named TS1 with a thickness dTsl and an effective refractive
index nT51
containing all layers between L1 and the entrance face. The spacer layer has a
thickness
selected from the range 20 - 30 m, the thickness dTSO in dependence on the
refractive index
nTSO and the thickness dTSl in dependence on the refractive index nTso are
within a specified
area. In this way a reliable read out of both the first and the second
information layer of
respectively LO and Ll is achieved.
